1. Field of the Invention
The present invention relates to an atomic probe type microscope apparatus, such as a scanning tunneling microscope apparatus, and, in particular, to an atomic probe type microscope apparatus which has an improved stability and controllability against a temperature variation.
2. Description of the Related Art
The scanning tunneling microscope (STM) utilizes a free electron wave (tunneling current) penetrated by a tunneling effect from the surface of an object to microscopically observe an array of atoms in the surface of the object while being scanned. The wavelength of the free electron wave is shortest among controllable wavelengths and is of the order of the interatomic distance of the object. It is possible to obtain data on a microstructure in the surface of the object with the use of the STM.
The STM includes a needle-tipped electrode for passing a tunneling current through a test specimen and a support for holding the needle-tipped electrode. An earlier type STM is normally of such a tube type that the needle-tipped electrode probe is provided on a cylindrical support. In recent years, an STM has been proposed, for example, in EP 0194323 Al, in which a small needle-tipped electrode is formed at an area of less than 1 mm.sup.2 by a microfabrication technique which has been employed in an LSI process. Such an STM so microfabricated will be hereinafter referred to as a ".mu.-STM".
A .mu.-STM is also known which can scan an object in a range of a few .mu.m with the use of an actuator. A cantilever type .mu.-STM which has been announced in the third STM International Conference is shown, as an example, in FIGS. 1A, 1B and 1C.
In FIG. 1A, reference numeral 1 shows a substrate which is preferably made of a semiconductor substrate such as silicon. X direction and Y direction actuators 2X and 2Y are formed on the top surface of the substrate 1. The actuators 2X and 2Y extend outwardly from the edge of the substrate 1 with their forward ends joined each other. A small needle-tipped electrode probe 3 is projected at a surface of the joined site of the actuators 2X and 2Y. Wires 4X to 6X, 4Y to 6Y and 7 are formed on the surface of the substrate 1.
FIG. 1B is a cross-sectional view showing actuators 2X and 2Y. As shown in FIG. 1B, the respective actuator is composed of a laminate comprising an SiO.sub.2 layer 8, Al layer 9, piezoelectric material layer 10, Al layer 11, piezoelectric material layer 12 and Al/Au layer 13. As the piezoelectric material layers 10 and 12, use is made of. for example, ZnO or lead zirconate titanate The Al layers 9, 11 and Al/Au layer 13 form electrodes for voltage application. A voltage is applied to the piezoelectric material layers through these electrodes. By so doing, a deformation, which is about 40 A/V in the case of using ZnO, is developed in the piezoelectric material layers 10, 12. By controlling the application voltage it is possible to control deformations in the actuators 2X, 2Y and to make a scan of a small needletipped electrode 3 in a predetermined, very small range.
FIG. 1C is a pan view showing a wiring state of the aforementioned cantilever type .mu.-STM. As seen from FIG. 1C, various wires are arranged on the surface of the substrate 1. The wires 4X, 5X and 6X are respectively connected to the electrodes 9, 11 and 13 of the actuator 2X and the wires 4Y, 5Y and 6Y are respectively connected to the electrodes 9, 11 and 13 of the actuator 2Y. The wire 7 is connected to the needle-tipped electrode 3. The wires 4X to 6X, 4Y to 6Y and 7 are connected to terminals 14X to 16X, 14Y to 16Y and 17, respectively.
In the fabrication of the cantilever type .mu.-STM, the SiO.sub.2 layer 8, Al layer 9, piezoelectric material layer 10, Al layer 11, piezoelectric layer 12 and Al/Au layer 13 are formed over the substrate 1 by means of, for example, a CVD, sputtering or PEP technique to provide the actuators 2X and 2Y over the substrate 1. Further, the small needle-tipped electrode 3 is formed at the joined site 3 of the actuators 2X and 2Y. Then the portion of the substrate 1 is etched away so that the aforementioned actuators 2X, 2Y extend outwardly from the edge of the substrate 1 as shown in FIG. 1A.
The aforementioned small needle-tipped electrode probe 3 is formed in a process as shown in FIGS. 2A, 2B and 2C. That is, a spacer layer 21 and Ti/W mask layer 22 are formed over the piezoelectric material layer 12 of which the actuator is formed. In this case, the spacer layer 21 is formed of a removable material such as Cu. An opening of the order of 5 .mu. is formed by the PEP method in the mask layer 22 and, with the mask layer as an etching mask, the spacer layer 21 is overetched to provide an undercut hole 23. Then as shown in FIG. 2B a needle-tipped electrode 3 is formed as a conical body in the undercut hole 23 by vacuum deposition of a metal, such as Ta. Then the spacer layer 21 is etched and, as shown in FIG. 2C, the mask layer 22 and overlying Ta layer are lifted off the piezoelectric material layer 12.
In order to achieve a stable, high resolution on the STM of an atomic scale order, it is important to adequately eliminate an adverse effect resulting from vibration and temperature variation.
The aforementioned .mu.-STM is excellent in eliminating vibration. That is, because of an apparatus of that reduced size, the resonance frequency of the tunneling probe unit goes high, resulting in a strong immunity against vibration.
Studies have been carried out to eliminate an effect resulting from the temperature variation on the ordinary STM, but not adequately on the .mu.-STM.